Optical sensors or image sensors as they will be referred to here are increasingly being included in more and more different types of electronic devices, such as digital cameras and camcorders, laptop computers, cellular phones, tablet computers, and so on. The demand for higher resolution image sensors has resulted in the need for increasing numbers of individual optical sensors or picture elements (“pixels”) that collectively form the image sensor. To maintain the overall size of the image sensor or even reduce the overall size when the image sensor is being placed in compact devices like a cellular phones, the size of the individual pixels has had to decrease dramatically as the resolution of image sensors has gone from 2 megapixels (MP) to 8 MP, 10 MP, 12 MP, 14 MP and beyond. Currently, image sensors having pixel sizes of 2 micrometers (μm), 1.75 μm, and 1.4 μm are being commercially manufactured and image sensors having a 1.1 μm pixel size are currently being developed and thus on the horizon of commercial availability.
As the size of the image sensors and corresponding pixels has continued to decrease, the yield loss of the image sensors during manufacturing has increased significantly. The smaller pixel size results in contaminate particles, such as dust particles, present during the manufacturing of the image sensor adversely affecting the performance of the image sensors. Dust particles that may have been relatively small compared to the size of individual pixels in older, lower resolution image sensors are now of such a size that these dust particles can impair the proper operation of pixels in an image sensor and thereby render that image sensor defective. Current data shows the yield loss (i.e., percentage of image sensors that do not meet required specifications) is 2-5% for image sensors having 1.75 μm pixels, 5-10% for image sensors having 1.4 μm pixels, and for image sensors having 1.1 μm pixels may reach upwards of 20%.
Image sensors are typically either complementary metal-oxide semiconductor (CMOS) or charge-coupled device (CCD) type sensors. During semiconductor manufacturing processes utilized to fabricate these types of image sensors contaminant particles such as dust particles naturally arise and are thus inherently present. As the resolution of the CMOS and CCD image sensors has increased the size of the corresponding pixels has decreased, meaning the size of dust and other contaminant particles has increased relative to the size of the pixels. As a result, these contaminant particles present during the fabrication of high resolution CMOS and CCD image sensors can in some way adversely affect the operation of more pixels in the image sensor. Such adverse operation of more pixels in the image sensor can result in the image sensor failing functionality testing of the image sensor during manufacturing, which ultimately results in rejection of the image sensor. The more image sensors that are rejected the higher the yield loss, and as discussed above as the size of the pixels decreases the yield loss increases due at least in part to the increased relative size of the contaminant particles relative to the pixels. Moreover, it should be noted that contaminant particles can also lower the overall performance (i.e., lower the quality of image capture) for the image sensor even though the image sensor passes functionality testing.
These dust and other contaminant particles are inherently generated during the manufacturing process of image sensors. For example, a large number of image sensors are formed in a semiconductor wafer as is conventional in formation of integrated circuits, as will be understood by those skilled in the art. Once all required process steps have been completed on a semiconductor wafer, the individual image sensors formed therein are separated through it is typically referred to as a “dicing process” or “singulation” and once separated each of the individual pieces is referred to as a “die,” as will also be understood by those skilled in the art. This dicing process typically involves sawing the semiconductor wafer to form individual dies and thus creates sawing dust particles that can contaminate the image sensors formed on the respective dies. The individual dies must then be attached to some package substrate or carrier which can involve the generation of more sawing particles and also involves glue that is used to attach the die to the carrier and which can also contaminate or damage the image sensor. Furthermore, wire bonding steps involve electrically connecting the package carrier and the die and can also result in contamination or damage to the image sensor formed on the die. The same is true when a suitable lens is attached to the package carrier. During all these operations, inherent particles present in a “clean room” in which the image sensors are being fabricated can contaminate or damage the pixels of the image sensors on the respective dies. Clean rooms are extremely expensive facilities and thus efforts to reduce the contaminant particles that are present by improving the clean rooms are very costly. Note that in the following description the terms “contamination,” “contaminant,” and “contaminant particle or particles” will be used interchangeably to broadly to refer to any type of particle that interferes with the proper operation of an image sensor, whether by damaging the image sensor itself or by impairing the proper operation of the image sensor simply by being present on the image sensor.
There is a need to maintain or reduce the yield loss and quality of image capture for image sensors as the size of pixels forming such image sensors continues to decrease.